1. The present invention relates to power-line-operated power-factor-corrected electronic ballasts for gas discharge lamps, particularly of a type where the current drawn by these ballasts from the power line has a low percentage of total harmomic distortion, while the lamps receive lamp current of relatively low crest factor.
2. Description of Prior Art
There are two predominant types of electronic ballasts for gas discharge lamps: (a) a first type may be referred-to as the parallel-resonant type and involves the use of a current-excited (i.e., parallel-excited) parallel-loaded resonant L-C circuit; and (b) a second type that may be referred-to as the series-resonant type and involves the use of a voltage-excited (i.e., series-excited) parallel-loaded resonant L-C circuit.
An example of the parallel-resonant type of electronic ballasts is described in U.S. Pat. No. 4,277,726 to Burke. An example of the series-resonant type of electronic ballasts is described in U.S. Pat. No. 4,538,095 to Nilssen.
Of these two types of electronic ballasts, the parallel-resonant type is conducive to yielding a stable easy-to-control self-oscillating inverter-type ballast; whereas the series-resonant type, although potentially simpler and more efficient, is harder to control in that it has a natural tendency to self-destruct in case the lamp load be removed and/or in case the phasing between applied voltage and resulting current were to become such that the current be leading.
To mitigate this tendency to self-destruct under no-load and/or special load conditions, various protection circuits have been developed, such as for instance described in U.S. Pat. No. 4,638,395 to Nilssen.
However, in situations where these types of electronic ballasts are powered from the AC voltage on an ordinary electric utility power line, it is important to assure that the current drawn by the ballast from the power line is drawn with a high power factor (preferably at least 90.0%) as well as with a low amount of total harmonic distortion (preferably no higher than 20.0%). Also, it is important that the gas discharge lamp be powered with a lamp current having a relatively low crest factor (preferably no higher than 1.7).